The present invention relates to composite materials comprising an inorganic matrix in which are distributed inclusions of carbon-containing material, constituted by inclusions of vitreous carbon and/or graphite and/or carbides.
Such composite materials can be used in numerous fields and in particular in the mechanical and electrical engineering industries as electrical contact and/or friction materials.
More specifically, the invention relates to a process for the preparation of composite materials incorporating inclusions of vitreous carbon having controlled dimensions and distributed in a substantially uniform and regular manner within a preferably metallic, dense or porous inorganic matrix.
It is pointed out that vitreous carbon is an artificial variety of the carbon obtained by pyrolysis, under strictly controlled conditions, of crosslinked polymers obtained by the polycondensation of phenols and aldehydes. The carbon obtained during this pyrolysis is called vitreous carbon due to its appearance, hardness, fragility (comparable to that of porcelain) and impermeability to gases comparable to that of glass.
In order to obtain this vitreous carbon, pyrolysis of the crosslinked polymer is carried out in such a way that the three-dimensional lattice of the polymer is retained throughout the operation and gives this special structure to the end product.
Vitreous carbon has interesting properties for various applications. Thus, bearing in mind its special structure defining micropores, which are not in communication with one another and which do not issue onto the surface of the material, the latter has a low density of approximately 1.45, whereas that of graphite is 2.2. Moreover, its mechanical properties are close to those of pyrolytic graphite or pyrocarbon. It has a thermal conductivity of approximately 0.04 to 0.08 Joule/cm.sup.-1..degree.C..sup.-1.s.sup.-1 and an expansion coefficient of approximately 3 to 5.10.sup.6..degree.C..sup.-1 at 100.degree. C. and 20.10.sup.-6..degree.C..sup.-1 at 1500.degree. C., so that it has a remarkable resistance to thermal shocks. Moreover, vitreous carbon has a resistance to oxidation which is higher than that of other varieties of carbon and graphite, whilst more particularly having a good resistance to oxidation by oxygen, water vapour or carbon dioxide gas.
It is also possible to improve the mechanical, thermal and/or chemical properties of certain inorganic materials by adding vitreous carbon to them. For many years, attempts have been made to produce composite materials having either vitreous carbon inclusions or a vitreous carbon covering. However, the processes used up to now have not made it possible to obtain a composite material having an inorganic matrix in which vitreous carbon inclusions with controlled dimensions are dispersed in desired proportions.